Steam power plant with forced-flow boiler system, particularly for supercritical pressure, and a superimposed circulating system



May 28, 1968 R. MICHEL 3,385,270

STEAM POWER PLANT WITH FORCED-FLOW BOILER SYSTEM, PARTICULARLY FORSUPERCRITICAL PRESSURE, AND A SUPERIMPOSED CIRCULATING SYSTEM Filed Feb.5, 1967 United States Patent 0 3,385,279 STEAM POWER PLANT WITHFORCED-FLGW BGILER SYSTEM, PARTICULARLY FOR SUPER- CRITICAL PRESSURE,AND A SUPERIMPOSED @IRCULATKNG SYSTEli i Ruppreeht Michei, Erlangen,Germany, assignor to Siemens Aktiengesellschatt, Berlin, Germany, acorporation of Germany Continuation-impart of application Ser. No.445,427, A r. 5, N65. This apphcation Feb. 3, 15367, Sci. No. 513,793

4 Qlaims. (Cl. 122-4ti6) ABSTRACT OF THE DKSQLOSURE A temperatureregulating system is combined with a steam power plant having aforced-flow system with a series-connection of working-*ncdium supplysource, a preheater, an evaporator, 21 superheater having a finishingsuperheater surface, a load and intermediate superheater, and acirculating system superimposed upon the forcedfiow system between ahigh-temperature point arrear of the evaporator and a low-temperaturepoint ahead of the evaporator. The circulating system contains animpeller pump for withdrawing medium from the forced-flow system at thehigh-temperature point and recycling the medium back to thelow-temperature point at least during start-up and partial-loadoperation of the plant. The regulating system includes a regulator whichsimultaneously regulates the steam temperature at the intermediatesuperheater and the temperature of the recycled medium at the pump. Theregulator has a heat exchanger connected to the intermediate superheaterand connected in the circulating system for cooling the recycled mediumby steam substantially at the intermediate superheater temperature andan injector connected downstream of the low-temperature point whichsupplies feed-water therefrom to the circulating system.

The instant application is a continuation-in-part of my copendingapplication Ser. No. 445,427, filed Apr. 5, 1965, now abandoned, whichis based upon German application S 84,554, filed Apr. 4, 1963.

My invention relates to a steam power plant with a forced-flow boilersystem upon which a circulating sy tem is superimposed in such a mannerthat the circulating system withdraws working medium from theforced-flow system at a suitable point of relatively high temperature,for example between preheater and final superheater, and recycles thewithdrawn medium back into the forcedflow system at a point of lowertemperature preceding the evaporator in the flow direction and located,for example, between the economizer heating surfaces and the evaporator.The circulating system may be thus super imposed upon the forced-flowsystem for any type of plant operation, or preferably only duringstartup and partialload operations.

In such a combined forced-flow and circulating system, care must betaken that the impeller pump in the circulating system always deliversthe working medium in liquid condition. This, in many cases, leads tothe necessity of cooling the working medium supplied to the pump inorder to secure a reliable pumping operation.

It is an object of my invention to provide a system which reliablysecures proper cooling of the partial flow of working medium passingthrough the circulating system in a combined forced-flow and circulatingsystem of the abovementioned kind under any occurring operatingconditions.

Another object of the invention is to improve the operation of suchcombined systems by establishing and Fatented May 28, 1968 maintaining afavorable correlation between the cooling of the recirculating workingmedium and the temperature of the generating steam.

To achieve these objects, and in accordance with a feature of myinvention, the cooling of the working medium withdrawn from, andrecycled back into, the forced-flow system is effected to a large orpreponderant portion with the aid of intermediate-superheater steam. Asa result, the invention affords cooling the circulating quantity ofworking medium simultaneously with utilizing this operation for thetemperature regulation of the steam superheated in one or moreintermediate superheaters (reheaters).

According to another feature of the invention, the cooling of therecycled working medium in the circulating system of the plant iseffected by passing the medium through one or more heat exchangers inwhich they are thermally coupled with a flow of steam coming from one ormore intermediate superheaters.

According to still another feature of the invention the medium in thecirculatin system is additionally cooled by coolant supplied from plantcomponents other than the intermediate superheaters. Preferably suchadditional cooling is employed in cases where the regulating range forthe intermediate superheating of the steam does not sutfice forsatisfactorily cooling the entire quantity of recycled working medium.

According to another, more specific feature of the invention, theadditional cooling of the medium in the circulating system is effectedby injection of water. Applicable for this purpose is feed waterwithdrawn at a suitable locality arrear of the feed-water pump. Inplants which have an economizer connected between the preheater portionand the evaporator portion of the forcedflow system, the feed water ispreferably withdrawn from this system at a locality between thepreheater and the economizer heating surface, this locality of waterwithdrawal being thus situated ahead of the low-temperature point atwhich the working medium is recycled back into the forced-flow system.

The invention will be elucidated more in detail with reference to theaccompanying drawing illustrating a schematic circuit diagram of a powerplant according to the invention by way of example.

The illustrated embodiment comprises a once-through boiler operating atsupercritical pressure. The working medium, in the form of feed water,is supplied from a condensator 1 by a condensate pump 2 from which itpasses through a low-pressure preheater portion of which two stages 3and 4 are represented. These two preheater stages 3 and 4 are connectedby the lines 50 and 51, respectively, with a low pressure stage 18 of asteam turbine 14. The preheater water then enters into a feedwater tank5. The feed-water pump 6 for withdrawing the water from the tank 5 israted for supercritical pressure, although the boiler operation may bein the subcritical range during startup and shutdown of the boiler and,if desired, also during partial-load operation.

On the high-pressure side of the forced-flow system, the working mediumpasses from the pump 6 through an inlet valve 52 to a high-pressureregenerative preheater portion of which, for simplicity of illustration,only two high-pressure preheater stages 7 and 8 are shown on thedrawing. The preheater stage 7 is connected through the line 54- 'withthe steam outlet line 53 of a medium-pressure stage 17 of the turbine14, and the preheater stage 8 receives heatin steam through the lines 57and 58 from the steam outlet line 56 of a high-pressure stage 16 of theturbine 14. The preheater portion is followed by an economizer 9 towhich there are connected in series the evaporator stage 10 and aheating surface 11 which may operate as a post-evaporator. However, theheating Sl11'-- face 11, depending upon the particular operating mode ofthe plant, may also be looked upon as being a presuperheater heatingsurface, in which case the next following heating surface 12 constitutesthe finishing superheater. If desired, and as illustrated, a pressureholding valve 13 may be inserted ahead of the superheater end stage 12.

The superheated fresh steam passes through a line 19 and an inlet valveto the steam turbine 14 driving an electric generator 15. The steamturbine comprises the aforementioned high-pressure stage 16,medium-pressure stage 17, and low-pressure stage 18. After leaving thehigh-pressure turbine 16 the steam is reheated in a first intermediatesuperheater (reheater) 21. The reheated steam then passes into themedium-pressure stage 17. A second intermediate superheater 22 connectsthe output of the turbine stage 17 with the input of the low-pressurestage 18 from which tthe steam then passes through the line 60 to thecondenser 1.

The above-mentioned superimposed circulating system comprises a line 23which branches off the forced-flow system at a point of relatively hightemperature ahead of the finishing superheater 12. The working medium(steam) withdrawn through the line 23 passes through a check valve 24into a first heat exchanger 25 in which a condensation of the workingmedium takes place. The coolant in exchanger 25 is constituted by thewaste steam, or a partial current of the waste steam, coming from thehigh-pressure turbine stage 16 through the lines 56, 37 and 59. In heatexchanger 25 the condensed medium passes through a connecting line 25into a second heat exchanger 26 in which it is further cooled by steamissuing from the medium-pressure turbine 17 and conveyed thereto throughthe lines 53, 38 and 55. If the circulating line 23 is branched oil at apoint of the forced-flow system having a temperature of 400 C. forexample, the circulating quantity can be readily kept at approximately300 C. in the line 27 of the circulating system behind the heatexchanger 26. Consequently when operating with supercritical pressure,the impeller pump 28 connected in the circulating system is notsubjected to higher maximum temperatures than about 370 C. In theillustrated embodiment, however, an injection cooler 29 is interposed inthe circulating system ahead of the impeller pump 28 so that it receivesfeed water at a temperature of about 300 C. or less.

The automatic temperature regulation is effected by means of a feed-backcontrol system described presently.

A main regulator 30 receives temperature datum values indicative of thetemperature which the reheated steam in the intermediate superheaters(reheaters) is to maintain. Since the illustrated example is providedwith two reheaters 21 and 22, two corresponding heat sensors 31 and 32are provided to furnish respective pilot signals indicative of the steamtemperature at the reheaters 21, 22, thus affording a separatetemperature regulation for each of the reheaters in accordance withselected datum values respectively.

The temperature for the first reheater 21 is maintained at the selecteddatum value by a feed-back regulator 33 of any suitable conventionaltype in which the pilot signal from sensor 31 passing over theconnecting line 61 is compared with the selected datum value, and theregulator output controls a three-way valve through the connecting line62 in accordance with the amount and direction of any departure betweenpilot and datum values. The temperature of the reheated steam behind thesecond reheater 22 is maintained at the selected value by means of theregulator 34 which receives its measured value from the location 32through the line 63 and operates in an analogous manner to the regulator33. The two regulators 33 and 34 are subordinated to the main regulator30 and are connected thereto by the signal lines 64 and 65,respectively.

The regulation of the flow rate of steam through the heat exchangers 25and 26 and through the preheater stages 7 and 8 in the illustratedembodiment is performed by operation of the above-mentioned valve 35 anda corresponding valve 36, which are located at the junctionsrespectively of steam lines 57, 58, 59 and 53, 54, 55.

The main regulator is also connected through a signal line 66 withanother subsidiary regulator 39 which receives temperature signalsthrough a signal line 67 from a sensor 40 located in the circulatingsystem ahead of the impeller pump 28. For additionally cooling therecycled quantity of working medium, the regulator 39 controls valves inone or more injection lines 41, 42, 43 and 44. The illustrated examplerepresents the possibility of taking the quantity of injection waterfrom the forced-flow system at a point ahead of the high-pressurepreheater 7 under control by the valve 45, and also at a point ahead ofthe high-pressure preheater 8 under control by the valve 46, and furtherat a point ahead of the economizer 9 under control of the valve 47. Theregulation is readily adaptable, either to required operating conditionsor to highest permissible values, with the aid of conventional means sothat, on the one hand, predetermined steam temperatures are maintainedor not exceeded at the outlets of the intermediate superheaters and, onthe other hand, the temperature of the recycled working medium ahead ofthe impeller pump 28 is maintained at or below prescribed values.

The main regulator 30 in the illustrated system may be constituted byany conventional boiler-load controller or a component thereof andreceives datum commands as to the power to be delivered by the powerplant, these datum inputs being set either manually, or automaticallyfrom the power distribution system served by the power plant, or alsofrom monitor or programming apparatus operating in accordance with apredetermined working program. The required power-output values thuscommanded then correspond to respective temperature and pressure valuesof the working medium at the various inputs and outputs, as well as thetaps of the turbine stages and appertaining heating surfaces. Of theseindividual datum values only those referring to the temperature datumvalues for the reheated steam in the intermediate superheaters are ofinterest to the present invention.

As mentioned, the regulators 33, 34 and 39 are subordinate to the mainregulator 30. In the illustrated system embodiment, the reheated steamtemperature in the first intermediate superheater 21 and the temperaturein the second intermediate superheater 22 are to be regulated, and itmay happen under certain operating conditions, or generally, that thereheated steam temperature in the two intermediate superheaters mustassume different prescribed magnitudes respectively. This is the reasonWhy in the illustrated system embodiment two separate regulators 33 and34 are provided for the intermediate superheaters. Although, as shownand described above, both subsidiary regulators 33 and 34 are connectedto the main regulator 30 to receive the same temperature datum valuetherefrom, it is the purpose of the subsidiary regulators 33 and 34 toperform in each case a modifying control which maintains the temperatureof each intermediate superheater at the particular datum value distinctfrom that of the other intermediate superheater.

If the temperatures of the two intermediate superheaters 21 and 22,sensed at the respective localities 31 and 32, are too high, theregulators 33 and 34 issue to the respective three-way valves 35 and 36corresponding control signals through the signal lines 62 and 68. Thesevalves then decrease the flow through the lines 59 and 55 from the heatexchangers 25 and 26 so that a larger amount of steam flows from theturbine outlet lines 56 and 53 through the lines 37 and 38,respectively, directly to the respective intermediate superheaters 21and 22. If the steam temperature is too low at the intermediatesuperheaters 21 and 22, the three-way valves 35 and 36 receive a signalto increase the flow through the heat exchangers 25 and 26 by openingthe junctions of the lines 59 and 55 so that a smaller amount of steamflows through the intermediate superheaters 21 and 22. The more thevalves 35 and 36 are closed and the steam flow through the respectiveheat exchangers 25 and 26 is reduced, the more steam is permitted todirectly reach the intermediate superheater 21 or 22 through the lines56, 37 and 53, 38, respectively.

The regulator 39, also subordinate to the main regulator 30, receivesfrom the main regulator through the signal line 66 the same main datumvalue as the other subordinate regulator and compares it with the real(pilot) temperature at the sensing location 40. When a sufiicientdeparture is thus determined, the regulator 39 controls the injectioncooler 29 to become elfective to a greater or lesser extent by changingthe setting of the valves 45, 46 and 47. The operation is preferablysuch that the heat exchangers Withdraw from the circulating system asmuch heat as is required for regulating the reheated steam temperatureand the heat exchangers are rated, in relation to the heating of theintermediate superheaters, to leave only a small residual regulatingrange for operation of the injection cooler 29.

If desired, the sensing location 40 may also comprise a pressure-sensingdiaphragm or other pressure gauge which transmits a correspondingpressure signal to the regulator 39. This is in accordance with the mainpurpose of the regulator 39 to always secure a sufiiciently lowtemperature of the circulating medium so as to keep it liquid ratherthan vaporous, at any obtaining pressure of the Working medium. As arule, the regulator 39 need effect a superpositional control operationonly when the cooling of the circulating working medium in the heatexchangers 25 and 26 no longer suffices to prevent the danger that thepump 28 may receive an excessive share of steam. The regulator 39,therefore, may operate as a limit-responsive regulator and becomeelTective only if the temperature at the sensing locality 4t? approachesthe evaporation temperature at the obtaining pressure.

To those skilled in the art, it will be obvious upon a study of thisdisclosure, that my invention permits of various modifications, forexample with respect to layout and number of system components, andhence may be given embodiments other than particularly illustrated anddescribed herein, without departing from the essential features of myinvention and within the scope of the claims annexed hereto.

I claim:

1. With a steam power plant having a forced-flow system with aseries-connection of working-medium supply means, a preheater, anevaporator, superheater means having a finishing superheater surface, aload and intermediate superheater means, and having a circulating systemsuperimposed upon said forced-flow system between a hightemperaturepoint arrear of said evaporator and a lowtemperature point ahead of saidevaporator, said circulating system containing an impeller pump forwithdrawing medium from said forced-flow system at said hightemperaturepoint and recycling said medium back to said low-temperature point atleast during start-up and partial-load operation of the plant, thecombination of a temperature regulating system comprising regulatingmeans for simultaneously regulating the steam temperature at saidintermediate superheater means and the temperature of the recycledmedium at said pump, said regulating means comprising heat exchangermeans connected to said intermediate superheater means and connected insaid circulating system for cooling the recycled medium by steamsubstantially at the intermediate superheater temperature and injectionmeans connected downstream of said low-temperature point for supplyingfeed-water therefrom to said circulating system.

2. In a steam power plant with a regulating system according to claim 1,said forced-flow system having an economizer interposed between saidpreheater and said low-temperature point, said injection meanscomprising a feed-water injection line branching off said forced-flowsystem at a locality ahead of said economizer to receive a flow offeed-water, said line having an injection outlet in communication withsaid circulating system for additionally cooling the recycled medium andhaving injection control means connected to said regulating means to becontrolled thereby.

3. In a power plant with a regulatin. system, according to claim 1, saidregulating means also including control valve means interposed betweensaid load on the one hand and both said intermediate superheater meansand said heat exchanger means on the other hand, temperature sensormeans responsive to the steam temperature at said intermediatesuperheater means, and control means responsive to said sensor means andconnected to said valve means for controlling said valve means independence up on said steam temperature at said intermediate superheatermeans.

4. In a power plant with a regulating system, according to claim 3, saidwater injection means communicating with said circulating system betweensaid heat exchanger means and said pump, said injection means havinginjection valve means for varying the amount of water injected into saidcirculating system, said regulating means also including further controlmeans having temperature sensor means responsive to the temperature ofthe medium in said circulating system at a point between said pump andsaid water injection means, said latter control means being connected tosaid injection valve means for controlling the latter.

References Cited UNITED STATES PATENTS 3,129,564 4/1964 Brunner 1224063,183,896 5/1965 Lytle et al 122406 3,186,175 6/1965 Strohmeyer 122-479XR KENNETH W. SPRAGUE, Primary Examiner.

